This invention relates to nuclear reactors and has particular relationship to fuel assemblies. A fuel assembly includes a skeleton in which fuel rods are supported and into which control rods and the like penetrate. The skeleton includes an upper or top nozzle, a lower or bottom nozzle and a plurality of egg-crate-like grids spaced between the nozzles. The grids are composed of interlaced straps. The nozzles and straps are held together by thimble tubes (some of which receive the control rods) which extend between the nozzles and to which the grids are secured. Each fuel rod is held in a column of aligned pockets in the grids between oppositely-disposed springs and dimples. The springs are each mounted on a part of a strap bounding a pocket and urge the rods into firm engagement with the dimples in the part of the opposite strap bounding the jacket. The fuel assemblies are mounted between the upper and lower core-support plates of the reactor.
This invention is described in this application as integrated into a nuclear reactor of the pressurized water type (PWR) to which it is uniquely applicable. It is to be understood that embodiment of this invention into reactors of other types is within the scope of equivalents of this invention.
During normal operation of a PWR, the flow of coolant, which may be as high as 50 feet per second, through each fuel assembly produces a net upward force of substantial magnitude on the assembly that would cause vertical movement, i.e., would cause the assembly to rise, if the assembly were not restrained. In accordance with the teachings of the prior art, that restraint is provided by a plurality of heavy leaf springs mounted on the top nozzle. These springs are compressed by the upper core plate and produce a restraining force that is larger than the assembly lift forces by a specified magnitude. The force which the springs counteract may be as high as 1500 to 2000 pounds. The compressive load supplied by the springs varies over the assembly lifetime because of thermal and irradiation-induced differential growth of the assembly relative to the upper core plate and spring irradiation-induced relaxation.
The leaf springs are costly and complicate the structure and use of the top nozzle and of the fuel assembly as a whole. They constitute an appreciable increase, formidable in its demands, of the number of parts which must be assembled to construct a fuel assembly and maintained during its life. Because the springs are mounted on the corners of the upper nozzle but engage the core plate a distance from the corners, the upper nozzle must have substantial depth so that it can withstand the high bending loads exerted by the springs. The length of the fuel rods which can be accommodated by the prior-art assembly is correspondingly reduced. The leaf springs exert a reactive upward load on the upper core plate and on the upper internals of the reactor even when the coolant is not flowing as, for example, after refueling when the vessel head has been reinstalled. This upward load exacerbates the difficulty of tensioning the bolts which secure the head to the body of the reactor pressure vessel following refueling or initially. The resilience and dimensions of the leaf springs are materially affected by the thermal conditions in the reactor and by neutron irradiation. The downward load on the lower core-support plate, which is impressed solely by the leaf springs, thus varies in normal operation as the temperature within the reactor varies and also changes progressively, during the life of the reactor as a result of neutron irradiation.
It is an object of this invention to overcome the above-described disadvantages and drawbacks of the prior art and to dispense with the heavy holddown springs in the fuel assemblies of a nuclear reactor. It is an object of this invention to provide a nuclear fuel assembly for a nuclear reactor which shall not include the heavy holddown springs of prior-art assemblies.